Failure modes in physical structures occur in several different ways, but generally always begin in a small fashion and increase with time. Detection of failure early can prevent costly damage due to a completed failure. However, the cost of inspection, including labor and downtime often mitigates in favor of allowing total breakdown rather than the desired preventative maintenance.
Any system which could detect the onset of failure would be of great benefit in preventing the cost associated with total failure to the point of destruction. Further, a system of sufficient resolution could monitor smaller failures and permit replacement of the part at a point just before the failures become large enough to warrant a danger of destructive failure.
Any such monitoring system, to be worth its employment, would have to be low cost, require as little inspection time as possible, and have a high resolution. Ideally such a system would be selectably operable as a passive or active sensor. Fiber optic systems generally have appeared in the literature and have been used in a variety of ways.
For example, U.S. Pat. No. 4,743,116 to Udd et al., and assigned to the same assignee of the instant application, entitled "Microbending Optical Fiber Accelerometer and Gyroscope" discloses a fiber optic transducer for accelerometers and gyroscopes that utilize microbending of an optical fiber between two suspended points by vibrating the optical fiber with electro-static deflection to induce modulation of a light signal to sense acceleration, tension, etc.
U.S. Pat. No. 4,295,738 to Meltz et al., entitled "Fiber Optic Strain Sensor" discloses an optical fiber having at least two cores positioned in a common cladding that can be fabricated to be responsive to strain or hydrostatic pressure but not to temperature through the selection of materials, spacing and shape of the cores and cladding in the fiber.
U.S. Pat. No. 4,301,543 to Palmer, entitled "Fiber Optic Transceiver and Full Duplex Point-To-Point Data Link" discloses a full duplex data link for use between two transceiver terminals utilizing a single optical fiber for data transmission.
U.S. Pat. No. 4,408,495 to Couch et al., entitled "Fiber Optic System For Measuring Mechanical Motion Or Vibration Of A Body" discloses a system for monitoring vibration or mechanical motion of equipment utilizing an optical waveguide sensor coupled to the equipment.
U.S. Pat No. 4,421,979, U.S. Pat. No. 4,459,477, U.S. Pat. No. 4,463,254, and U.S. Pat. No. 4,477,725 to Asawa et al., entitled "Microbending Of Optical Fibers For Remote Force Measurement" discloses a system for remote measurement of structural forces including a plurality of microbend transducers mounted along the length of the structure for microbending an optical fiber in response to structural forces.
U.S. Pat. No. 4,530,078 to Lagakos et al., entitled "Microbending Fiber Optic Acoustic Sensor" discloses a microbending fiber optic acoustic sensor. A point hydrophone according to a first preferred embodiment of the present invention includes two opposed pistons which are subjected to an acoustic pressure wave.
U.S. Pat. No. 4,632,513 to Stowe et al., entitled "Method of Making A Polarization-Insensitive, Evanescent-Wave, Fused Coupler With Minimal Environmental Sensitivity" discloses an optical coupler and method of making same. The coupling ratio of the coupler is polarization-insensitive. The optical coupler described is made from single-mode optical fibers. Each optical fiber has a length of nearly exposed core which is fused to the exposed core of the other optical fiber while the fibers are maintained in parallel juxtaposition with one another without twisting.
U.S. Pat. No. 4,657,659 to Rempt et al., entitled "Fiber Optic Displacement Sensor" discloses an optical displacement sensor that serves to sense variations in the displacement of a test article.
U.S. Pat. No. 4,678,903 to Wlodarczyk et al., entitled "Self Aligning Fiber Optic Microbend Sensor" discloses a self aligning microbend sensor comprising a tubular housing having internal threads, an expandable insert with external threads loosely threaded into the housing with sufficient tolerance to allow expansion of the inert in response to a parameter being measured while maintaining registration of the threads, and a fiber optic element between the housing and the insert and extending transverse to the threads, such that upon expansion of the insert the fiber optic element experiences microbending due to deformation by the threads.
U.S. Pat. No. 4,734,577 to Szuchy, entitled "Continuous Strain Measurement Along A Span" discloses a fiber optic load measuring system, and method of constructing the same which comprises a light source, a fiber optic attenuator, a photodetector, and a signal processor.
U.S. Pat. No. 4,738,511 to Fling, entitled "Molecular Bonded Fiber Optic Couplers and Method of Fabrication" discloses a fiber optic coupler which is formed by heating juxtaposed optically flat surfaces formed on two length of optical fiber to the glass transition temperature.
U.S. Pat. No. 4,749,248 to Aberson, Jr. et al., entitled "Device For Tapping Radiation From Or Injecting Radiation Into, Single Made Optical Fiber, And Communication System Comprising Same" discloses optical radiation efficiently removed from, or injected into, single mode optical fiber at an intermediate point along the fiber.
U.S. Pat. No. 4,770,492 to Levin et al., entitled "Pressure Or Strain Sensitive Optical Fiber" discloses pressure or strain sensitive optical fiber having a core, at least a portion of which has a predetermined refractive index for transmitting light therethrough, a cladding adjacent the core having a refractive index which is less than that of the core and a concentric light transmission layer adjacent the cladding having a refractive index which is greater than that of the cladding.
U.S. Pat. No. 4,781,424 to Kawachi et al., entitled "Single Mode Channel Optical Waveguide With A Stress-Induced Birefringence Control Region" discloses a single mode optical waveguide having a substrate, a cladding layer formed on the substrate, a core portion embedded in the cladding layer, and an elongated member for applying a stress to the core portion or a stress relief groove for relieving a stress from the core portion in the cladding layer along the core portion.
U.S. Pat. No. 4,788,868 to Wilk, entitled "Strain Measurement Apparatus And Method" discloses a method and apparatus for measuring the relative movement of one end of a cylindrical member relative to the other end employs first and second strain-sensitive, parametric elements, such as optical fibers.
U.S. Pat. No. 4,792,689 to Peterson, entitled "Method For Obtaining A Ratio Measurement For Correcting Common Path Variations In Intensity In Fiber Optic Sensors" discloses a method for correcting common path variations in intensity in fiber optic chemical sensing devices using a device for spatially separating light of different wavelength regions and a dye system selected so that light passing back to a measuring system along the fiber optic sensor consists of two wavelength regions.
U.S. Pat. No. 4,795,232 to Persson, entitled "Fibre-Optic Cable Responsive To Microbending Forces" discloses a fibre-optic responsive to microbending and forming part of a device for measuring pressure in accordance with the principle of creating a periodic mechanical disturbance in the fiber.
U.S. Pat. No. 4,799,787 to Mason, entitled "Optical Fibre Radiation Pyrometer" discloses an optical pyrometer for a gas-turbine engine having a radiation receiving head located to view the blades of the engine and supply radiation from the blades to one end of a fibre-optic cable.
U.S. Pat. No. 4,800,267 to Freal et al., entitled "Optical Fiber Microbend Horizontal Accelerometer" discloses a highly sensitive microbend horizontal fibre-optic accelerometer having been conceived, particularly useful in seismological research where minute accelerations must be detected.
U.S. Statutory Invention Registration No. H550 to Hester et al., entitled "Apparatus For Sensing Axial and Tangential Forces Exerted On A Spool Of Datalink Filament" discloses an apparatus for sensing and measuring axial force and torsional force exerted on a spool of fiber optic datalink filament as the filament is drawn off the spool at rates of speeds comparable to that experienced in the flight of a missile.
However, useful the above fiber optic devices and methods may be, there is still a need for an inexpensive system, integral with the structure being monitored.